Apparatus for clamping plates to external drum devices, based on movable clamps

ABSTRACT

A system for loading and clamping one or more plates onto the cylindrical surface of a rotatable drum, such as used in a platesetter for printing, and for unloading the plates from the drum. The system is characterized by clamps that are movable over the surface, preferably along circumferential tracks, enabling the attachment of multiple plates, end-to-end and/or side-by-side, each plate having any of a wide range of sizes. The clamps are preferably of the dynamic type, whereby the clamping force increases with rotational speed. Also disclosed is a method for mounting and unmounting plates, using the movable clamps, such that allows flexible and convenient arrangement of loading- and unloading stations.

This is a Divisional of U.S. Ser. No. 09/735,798 filed Dec. 14, 2000,now U.S. Pat. No. 6,260,482, which is a Divisional of U.S. Ser. No.09/391,185 filed Sep. 7, 1999, now U.S. Pat. No. 6,189,452, which is aContinuation-in-Part of U.S. Ser. No. 09/069,546 filed Apr. 30, 1998,now U.S. Pat. No. 6,003,442.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to attaching a plate to a rotatable drumand, more particularly, to conveniently attaching plates of varioussizes to the drum of a so-called external-drum imagesetter orplatesetter, by means of movable dynamic clamps, such that enable it tooperate even at very high rotation speeds.

Imagesetters, namely marking engines for plotting images on plates orsheets of film, of the so-called external-drum type, are known in theart. Typically, a sheet is attached to the outer surface of a rotatingdrum and held in contact with the surface by a vacuum system, which hasorifices distributed over the surface. Often, such an imagesetter isrequired to handle sheets of various sizes. U.S. Pat. No. 5,383,001 toBosy addresses such a need, by providing a suitable design of the vacuumsystem.

External-drum platesetters are a particular category of external-drumimagesetters, wherein the image is plotted on a printing plate. Printingplates are generally made of metal and therefore they are stiffer thanfilms and their mass per unit area is much higher. Thus, when a plate isattached to the drum, it requires a greater force to keep it in contactwith the drum's surface at any given rotational speed, to counteract thecentrifugal force, than is required by a film. Consequently, a vacuumsystem to hold plates on the rotating drum must be more efficient andmore powerful than that used for holding films. However, even with apowerful vacuum system, there is a danger of failure, due to a bend or abulge at the edge of the plate or due to a failure in the vacuum systemitself. Moreover, above a certain rotational speed the centrifugal forceper unit area may exceed the difference between atmospheric pressure andthe actual vacuum pressure (which, in turn, has a practical lowerlimit), thus rendering such a plate attachment system, solely based onvacuum, ineffectual. In such a system failures may cause the entireplate to fly off the drum by centrifugal action—which would bedisastrous for the entire machine. Therefore platesetters requiremechanical attachment means—either to assist the vacuum system and guardagainst drastic failure, or as the main holding means, possibly stillaided by vacuum. One common and useful such means is clamps mounted onthe drum and operative to hold down the edges of the plate.

Clamps and grippers for holding a plate onto a rotatable drum have beenknown in the art. For example, clamps (also called grippers) aregenerally used in offset printing presses to hold the printing plateonto a cylinder. During operation, centrifugal forces act to cause apulling force on each edge of the plate, such that tends to pull theedge from the clamp. Therefore in printing presses, clamps are designedto exert enough gripping force to exceed that required for counteringthe centrifugally induced pulling force, at maximum expected rotationalspeed, by a safety margin. Additionally, the gripping force of theclamps must exert a positive stretching force on the plate, in order toassure contact between the plate and the cylinder over its entiresurface, as well as to counter the drag forces exerted on the plate byexternal rollers and cylinders that are in contact with it during theprinting operation. Such clamps are disadvantageously cumbersome formounting on platesetters and, moreover, difficult to activate (i.e. toopen for accommodating the plate), because they must include either aheavy bolting or leveraging mechanism or a very strong spring-loadingmechanism.

U.S. Pat. No. 3,203,074 describes an improved clamp, in which thegripping and stretching forces increase by centrifugal action inproportion to the rotational speed. A clamp with such a centrifugalaction (also termed dynamic clamp) may overcome the disadvantagesdiscussed in the previous paragraph. U.S. Pat. No. 4,250,810 discloses afurther improved clamp with centrifugal action, whereby the gripping andstretching forces can be independently adjusted.

It is noted that in platesetters, there usually is also a vacuum system,which assures contact between the plate and the drum over its entiresurface, and that therefore no stretching force, but only a grippingforce, is required at the edges of an attached plate. U.S. Pat. No.5,335,046 discloses a clamp for the drum of a platesetter with amechanism that centrifugally induces a gripping force. There is a row ofsuch clamps for the leading edge and another one for the trailing edge.The mechanism includes a weight, connected to the clamp by a rod,located inside the drum near the opposite surface. The clamp may beopened by means of a piston and cylinder assembly, activated by suppliedpressurized air. The clamp mechanism of the '046 patent has two seriousdrawbacks: (1) It is relatively complicated, with a relatively largenumber of parts, and thus is expensive to manufacture. (2) It does notconveniently accommodate plates of various sizes. The secondly listeddrawback also applies to other types of clamps of prior art, such asthose discussed hereabove. The '046 patent does disclose the possibilityof providing a row of trailing edge clamps for each plate size; however,such an arrangement is even more expensive and, furthermore, does notaccommodate small variations in plate size; moreover, it does notprovide for mounting a plurality of plates peripherally around the drum,in the case of small plates, thus detracting from productivity.

Associated with the manner of clamping plates to the drum is the mannerof loading them onto the drum and of unloading them following theplotting operation—whether manually or by automatic mechanisms. Mostsystems of prior art are limited in the loading- and unloadingconfigurations practically possible, generally enabling loading onlyupon, say, clockwise rotation of the drum and, correspondingly,unloading—only upon counterclockwise rotation of the drum. This placesconstraints on the design of loading- and unloading mechanisms and ontheir placement within the machine. More freedom in such design andplacement may, in many cases, be advantageous.

There is thus a widely recognized need for, and it would be highlyadvantageous to have, a clamping system, for attaching a plate to arotatable drum, that is adaptable to any plate size, possibly allowingthe attaching of multiple plates, of various sizes, and that enablesloading and unloading plates upon rotation of the drum in eitherdirection.

SUMMARY OF THE INVENTION

The present invention successfully addresses the shortcomings of thepresently known configurations by providing a an inexpensive dynamicclamping system, for attaching plates of various sizes to a rotatabledrum, such as that of a platesetter, so that they are firmly gripped atany rotational speed.

The present invention discloses a novel design of a clamp, which allowsplacing it at any position around the drum, to accommodate any length ofplate, and to be anchored at that position with a force that increasesby centrifugal action. The design of the clamp is, moreover, such thatthe gripping force exerted by it on the edge of the plate likewiseincreases with the rotation speed of the drum by centrifugal action. Thepresent invention further discloses a novel way of using such clamps toload and unload plates, to and from the drum, in the same generaldirection, thus enabling more practical configurations for feeding anddisposing plates.

More specifically and according to a preferred embodiment, the clamp ofthe present invention comprises a gripper that is attached to the drumby means of a hinge. The largest part of the gripper consists of a bodythat extends substantially to one side of the pivot axis of the hinge.The portion of the gripper that extends to the other side of the pivotaxis is relatively short and at its end is a tip, operative to press anedge of the plate onto the underlying surface of the drum. A spring isoperative to provide a static force for the tip's pressure on the plate.During drum rotation, centrifugal force virtually acting on the largebody of the gripper, in a radially outward direction, is translated bythe pivoting effect into a radially inward force on the tip, thusincreasing its pressure on the plate.

In a slidable configuration, the clamp also comprises a base, attachedto the other member of the hinge (the first member being attached to, orpart of, the gripper). The drum is provided with a plurality of undercutgrooves, preferably in a circumferential orientation, each groove beingwider at its bottom, inside the wall of the drum, than near the drum'ssurface. The base of the clamp is shaped to fit the wide portion of thegroove and to slide therealong. The gripping force of the clamp alsoacts to push the base against the upper surface of the groove's wideportion, causing friction therebetween, thus keeping the clamp fromsliding along the groove. The force of this friction also increases withthe drum's rotation speed, owing to the increased gripping force andowing to centrifugal action on the base itself.

Slidable dynamic clamps are preferably mounted in pairs around eachgroove, there being a plurality of grooves across the length of thedrum; the clamps of each pair face each other so as to clamp oppositeedges of a plate. Thus each pair of clamps along a groove can clamp downa plate, its length being variable over a wide range. Likewise, aplurality of plates, of various and variable widths, may be mountedacross the length of the drum, each plate clamped down by two or morepairs of clamps on adjacent grooves.

To mount a plate on the drum, a first clamp of each relevant pair isfirst opened, by pushing on its body radially inward by means of astationary activation mechanism, then engaging it to the leading edge ofthe plate, which is fed from a suitably positioned loading mechanism,and releasing the clamps to grip it. The drum is then rotated, sayclockwise, to pull the plate and wrap it around the drum. Then the otherclamp of each pair is opened and the drum is rotated backward (saycounter-clockwise), while the clamp remains stationary, until thetrailing edge of the plate is engaged by the clamp, whereupon it isreleased, thus gripping that edge by slidable clamps. After normalimaging operation, the plate is demounted, in the same general order, byfirst releasing the first clamp of each pair (which grips the leadingedge of the plate) and moving it away from the plate (by saycounter-Clockwise rotation of the drum), thus freeing that edge, thenrotating the drum forward (Clockwise), thus pushing the plate onto asuitably position unloading bin, and finally releasing the second clampof the pair, thus freeing the plate.

According to the present invention, there is provided apparatus forclamping flexible plates of various sizes onto the cylindrical surfaceof a rotatable drum, comprising:

a plurality of grooves, cut into the surface of the drum; and

at least one clamp, each attached to the drum at one or morecorresponding grooves and being switchable between two states—a clampingstate and a released state;

the clamp being fixed at any of a plurality of positions along therespective grooves and operative to clamp an edge of the plate, whichcrosses the groove, to the surface of the drum, when in a clampingstate, and being slidable along the respective groove, when in areleased state.

According to further features of the preferred embodiments of theinvention, each of the clamps is a dynamic clamp, whereby the force atwhich it clamps the plate, when in the clamping state, consists of aconstant component and a dynamic component, which is derived fromcentrifugal action and is essentially proportional to the rotationalspeed of the drum. Furthermore, each of the clamps includes:

a base that is slidable within, and along, the respective groove andforming one member of a hinge, having a pivot axis, and

a gripper, formed to have a tip and operative to press the edge of theplate against the drum,

the gripper being attached to the base so as to form the other member ofthe hinge and so that the center of mass of the gripper is at asubstantial distance from the pivot axis and in a first directiontherefrom that is generally opposite the direction of the tip from thepivot axis.

Additional features of the present invention include an activationmechanism, not attached to the drum and including at least one memberthat is movable essentially radially to the cylindrical surface andoperative—

to engage the gripper of any clamp, while in an appropriate position, ata point thereof that lies in the first direction from the pivot axis,

to switch the clamp to the release state, by moving radially inwards,and

to switch the clamp to the clamping state, by moving radially outwards.

In a preferred embodiment, the grooves lie along circumferential linesand the activation mechanism and any of the clamps, while in the releasestate and while being engaged by a respective one of the members,cooperate to cause the base of the clamp to slide along the respectivegroove in reaction to a rotation of the drum.

According to yet other features of the invention, the number of thegrooves is at least four, the apparatus being operative to clamp atleast two plates, side by side, onto the surface of the drum, the platesnot necessarily being equal in length or in width, and the number of theclamps in any groove is even, the clamps forming circumferentiallyadjacent pairs, the clamps of each pair being disposed and operative toclamp respective opposite edges of a corresponding plate.

In an alternative configuration of the invention, some or all of thegrooves lie along lines parallel to the axis of the drum.

In another aspect of the present invention, there is provided apparatusfor loading a flexible plate onto the cylindrical surface of a rotatabledrum, attaching the plate thereto and unloading the plate therefrom, theplate having a leading edge and a trailing edge, the apparatuscomprising:

a plate loading assembly and a plate unloading assembly, both positionedin the vicinity of the drum; and

at least one pair of sets of clamps, attached to the drum, all clamps ina first set of each pair being operative to clamp the plate at itsleading edge onto the surface of the drum and the second set of eachpair being operative to clamp the plate at its trailing edge onto thesurface of the drum;

and being operative to load the plate from the loading assembly, leadingedge first, while the drum rotates in a first direction, and to unloadthe plate onto the unloading assembly, leading edge first, while thedrum rotates in the first direction.

According to further features of the second aspect of the invention,only the first set of clamps clamp the plate during loading and only thesecond set of clamps clamp the plate during unloading, and each clamp isswitchable between two states—a clamping state and a released state—andcircumferentially movable, with respect to the surface of the drum,while in the released state.

According to still further features of the invention, the plate loadingassembly and the plate unloading assembly are positioned substantiallyat one and the same side of the drum or at the same level. In certainconfigurations of the invention, there are at least two pairs of sets ofclamps and the apparatus is further operative to similarly load at leastone other plate from the loading assembly and to attach them to the drumin circumferentially tandem positions, and a plate may have any of aplurality of widths and any of a plurality of lengths.

Also disclosed is a method for loading at least one flexible plate ontothe cylindrical surface of a rotatable drum, simultaneously clampingeach plate thereto and unloading each plate therefrom, each plate havinga leading edge and a trailing edge, the method comprising:

(a) providing a plate loading assembly and a plate unloading assemblyand positioning them in the vicinity of the drum;

(b) providing, for each plate to be simultaneously clampable to thedrum, a pair of sets of clamps and attaching them to the surface of thedrum so that each clamp is movable along a circumferential line, wherebyeach clamp is switchable between a clamping state and a released stateand being operative, while in the clamping state, to clamp an edge of aplate engaged thereto onto the surface of the drum;

(c) feeding a plate from the loading assembly until its leading edgeengages a first set of a corresponding pair of the sets of clamps, whileall clamps thereof are in the released state;

(d) switching the clamps of the first set, while engaged by the plate,to the clamping state, then slowly rotating the drum in a firstdirection so as to pull the plate from the loading assembly and to wrapit around the drum;

(e) moving the clamps of the second set of the pair, while in thereleased state, until they engage the trailing edge of the plate wrappedin step (d), then switching them to the clamping state;

(f) repeating steps (c), (d) and (e) for each additional plate to beloaded, if any;

(g) for unloading each clamped plate, switching the clamps ofcorresponding the first set to the released state and moving them awayfrom the leading edge, then slowly rotating the drum in the firstdirection so as to push the leading edge of the plate onto the unloadingassembly;

(h) switching the clamps of corresponding the second set to the releasedstate, to allow the unloading assembly to pull the plate, released instep (g), from the drum.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is an isometric drawing, schematically showing a drum with theclamping system of the present invention;

FIG. 2 is a cross-sectional view of the system of FIG. 1;

FIG. 3 is an isometric drawing, schematically showing a portion of thedrum of FIG. 1 with slidable clamps according to the present invention;

FIG. 4A is a side view and cross-sectional view of a slidable clampaccording to the present invention;

FIG. 4B is a top view and sectional side view of the slidable clamp ofFIG. 4A;

FIG. 5 is an isometric view of a stationary clamp according to thepresent invention;

FIG. 6 is a schematic isometric drawing of a platesetter illustratingoperation of the clamping system of the present invention.

FIG. 7 is an isometric drawing, schematically showing an alternativeconfiguration of the clamping system of the present invention.

FIG. 8 is a schematic end view of the clamping system of FIG. 6, withall clamps being slidable.

FIG. 9 is an end view of an alternative configuration of the system ofFIG. 8, schematically showing two pairs of clamps and plates clamped bythem.

FIG. 10 is an isometric drawing of a system similar to those of FIGS. 8and 9, showing an arrangement of multiple plates side-by-side.

FIGS. 11A and 11B are schematic end views of the clamping system of FIG.9, illustrating loading- and unloading plates according to a firstconfiguration.

FIGS. 12A and 12B are similar to FIGS. 11A and 11B, but illustrate apreferred configuration of plate loading- and unloading.

FIG. 13 is an isometric drawing of a clamping system of a configurationalternative to that of FIGS. 8 to 10.

FIGS. 14A and 14B is a schematic isometric drawing of a clamping system,showing additional slidable clamp types.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of a dynamic clamping system, for attaching oneor more plates to a rotatable drum and for enabling loading andunloading of the plates to- and from the drum upon its rotation ineither direction.

Specifically, the present invention can be used to easily attach aplate, or a plurality of plates, of any size to the drum of a markingengine, such as a platesetter, so that the plate is firmly gripped atany rotational speed. The clamping system is characterized by beingoperable for loading and unloading plates with the drum rotating ineither sense. While the preferred embodiment is concerned with attachinga printing plate to the drum of a platesetter (i.e. a device forplotting images from digital sources onto printing plates), it will beunderstood that the present invention is applicable to attaching alsoany other types of plates and sheets of any media, such as film, paperor plastic foils, to any rotatable drum for any other purpose. Thepreferred embodiment also includes clamps, or grippers, of aparticularly advantageous design, as described and claimed in acopending patent application. It should, however be understood that alsoother types of clamps and grippers may be used, such as those relyingentirely on springs, or on magnets, to provide the gripping pressure orcentrifugally actuated grippers of other designs—all coming within thescope of the present invention.

The principles and operation of a clamping system according to thepresent invention may be better understood with reference to thedrawings and the accompanying description.

Referring now to the drawings, FIG. 1 is an isometric view, and FIG. 2—across-sectional view, of a drum with a preferred embodiment of theclamping system of the present invention. There is shown a drum 10, withouter surface 20. Surface 20 is cut by two (generally—several) grooves12 running circumferentially and extending over a considerable portionof the drum's circumference. Each groove 12 has an undercut profile, tobe described herebelow. At one end of each groove 12 is a stationaryclamp 14, fixedly attached to drum 10. Stationary clamp 14 is attachedpreferably within a depression 22, cut into surface 20, in order to keepthe extent of the clamp above surface 20 to a minimum, so as to clear awriting assembly (not shown) suspended a distance above the surface. Ineach groove 12 there is a slidable clamp 16, to be described herebelow.Stationary clamp 14 and slidable clamp 16 are operative to grip a plate18 near its end edges 19. Drum 10 is preferably provided with a vacuumsystem, operative to press the entire surface of plate 18 onto surface20.

Referring now to FIG. 5, there is shown the construction of stationaryclamp 14. It consists essentially of two parts—a base 24 and a gripper26—mutually attached by a hinge 28, which, for example includes acylindrical pin 30, whose axis forms the pivot axis 32 of the hinge.Gripper 26 is formed so as to have a relatively large body 34, extendingto one side of pivot axis 32, and a relatively short tongue 36,extending to the other side of pivot axis 32. Body 34, preferably has aflat top surface, with a notch 37 cut across it. Tongue 36 has a tip 40.It is noted that base 24 serves to join hinge 28 to the drum and couldalso be regarded as part of the hinge structure; other structures andother means for such joining may be provided. The assembly of stationaryclamp 14 also includes a spring, such as coiled spring 38, disposedaround hinge pin 30.

Stationary clamp 14 is attached to drum 10 by fastening base 24 to thedrum, for example—by means of screws, preferably inside a depression 22(FIG. 1), so that tongue 36 points in a circumferential and so that tip40 lies beyond the depression (that is, in contact with surface 20).Spring 38 acts to lift body 34 away from the drum and thus press tongue36 toward the drum. The tip 40 of tongue 36 preferably has a frictionpad or one or more small protrusions thereon, facing the drum. Normally,tip 40 is pressed against drum surface 20. Spring 38 acts to make thispressure strong enough to grip the edge of a plate, which has beeninserted between the tip and the drum surface, under static conditions.All stationary clamps 14 are attached to drum 10 so that they arealigned along a line that, preferably, is parallel to the drum's axisand are all pointing in the same direction.

When drum 10 rotates, centrifugal force on body 34 tends to lift it awayfrom the drum. This force is translated by the pivoting action into aradially inward force of tip 40, which thus increases its pressureagainst drum surface 20 and consequently also the gripping force on theedge of a plate therebetween. The centrifugal force is essentially

CF=M×ω ² ×R

where M is the mass of gripper 26, ω is the angular speed of the drumand R is the distance from the drum's axis of rotation (not shown) tothe center of mass 42 of gripper 26. The force at tip 40 is essentially—

TF=CF×D 1/D 2

where D1 is the distance from pivot axis 32 to center of mass 42 and D2is the distance from pivot axis 32 to the middle of tip 40. Clearly, itis desirable, at any rotational speed, to make TF large enough to gripthe edge of the plate firmly enough to prevent its being pulled out.This can be achieved, for any given design of tongue 36 and tip 40(which also fixes D2), by making the moment of centrifugal force, CF×D1,to be of sufficient magnitude. This, in turn, is obtained, for a givendrum radius R and any angular speed ω, by making the moment of mass ofgripper 26 with respect to pivot axis 32, M×D1, to have sufficientmagnitude. The moment of mass may be made sufficiently large byappropriate construction of body 34; for example, M may be made largeenough by making the body's volume large and/or fabricating it out of ahigh-density substance, while a sufficient complementary value for D1may be obtained by shaping the body so as to extend sufficiently farfrom hinge 28.

We now refer to FIG. 4A, which shows slidable clamp 16 in a side viewand in a cross-sectional view and FIG. 4B, which shows the same in alength-sectional view and a top view. Slidable clamp 16 is, inprinciple, similar to stationary clamp 14—both in its construction andin its mode of action, except for some essential details, to bedescribed herebelow. It consists of base 44 and a gripper 46, joined bya hinge, which includes, for example, pivot pin 48. Gripper 46 is formedto have, on one side of pivot pin 48, an elongated massive body 50 and,on the other side of that pin, a short tongue 52, which ends with a tip60. A spring 51, preferably of the compression type, is disposed betweenbase 44 and body 50. When clamp 16 is mounted on drum 10 (as will bedescribed herebelow), spring 51 acts to push body 50 radially outwardsand, by the pivoting action, to press tip 60 radially inwards. When theedge of a plate lies under tip 60, the latter then presses against it.When the drum rotates, the centrifugal force virtually acting on body 50is added to the push of spring 51 and thus the pressure of tip 60 on theplate underneath it is increased—all in a manner similar to thatdescribed hereabove with respect to stationary clamp 14. Clearly, again,it is desirable to make the mass of body 50 of gripper 46 as large aspossible. This can be done by making it of dense material and/or bymaking any of its dimensions as large as possible. In the preferredembodiment described herein, body 50 is shaped to be narrow andrelatively tall (i.e. with large dimension in the radial direction), butalso other shapes are possible. According to this embodiment, base 44has a central slot sunk into its upper surface over most of its length,the width of the slot being slightly greater than the width of body 50.Thus base 44 can partially accommodate body 50 during the latter'smotion about pivot 48.

Referring now to FIG. 3, which shows a section of drum 10 withcorresponding segments of two grooves 12 (one of them shown cut open)and a slidable clamp 16 mounted inside each groove, it is noticed thateach groove 12 is undercut into the wall of drum 10; that is, the grooveis relatively narrow near surface 20 of the drum and wider under thesurface, inside the wall. Preferably, the profile shape of the wideportion 54 of groove 12 is rectangular. Base 44 of slidable clamp 16 isformed to conform to wide portion 54 and to easily slide therealong.Body 50 of gripper 46 is formed to be slightly narrower than the narrowportion 56 of groove 12, so that it is slidable through that narrowportion whenever base 44 slides through wide portion 54. Preferably,body 50 of gripper 46 is also formed, and clamp 16 assembled, so that inits normal position, body 50 protrudes from groove 12 beyond surface 20.The top surface of body 50 preferably has a depression, such as notch58, cut into it (to be engaged by an activation pin, as describedherebelow).

Slidable clamps 16 are mounted each in a groove 12 so that tip 60 oftongue 52 of gripper 46 points circumferentially in a direction that isopposite the direction in which the fixed clamps point. As will bedescribed herebelow, during normal operation, the (preferably) trailingedge of plate 18, mounted on drum 10, lies underneath tip 60 and ispressed by the latter. Tip 60 is preferably substantially wider thannarrow portion 56, thus bridging the latter and pressing the platedirectly against drum surface 20. It is also preferably provided, on itsside facing plate 18, with one or more small protrusions or with afriction pad. These are operative, under the aforementioned pressure bythe tip on the plate against surface 20, to firmly grip the edge of theplate.

The bottom surface of base 44 is preferably coated with a glide pad,which operates to ease its sliding on the bottom surface of wide portion54 of groove 12 whenever pushed against it. The top surface of base 44has preferably attached thereto one or more friction pads 62 (FIG. 4),positioned so as to be able to contact the top surface of wide portion54 of groove 12. The gripping force of tip 60 on the plate (due to theaction of spring 51 and, during drum rotation, due to centrifugal forceon body 50) is countered by an upward pressure of the portion of base 44that lies underneath (opposite) tongue 52. The base is thus pressedagainst the top surface of wide portion 54, preferably through pads 62,thereby inhibiting any sliding motion therebetween. During drumrotation, additional upward pressure is exerted by the entire base 44,due to centrifugal forces on itself—which adds to its friction with thetop surface of wide portion 54.

Looking at the action of slidable clamps 16 from a different point ofview, the clamps may be regarded as forcefully clamping together theplate (at its edge) and the outer shell of the drum (being between thetop surface of wide portion 54 and drum surface 20)—simultaneouslyanchoring the plate and base 44 to the drum. The clamping force isincreased by centrifugal action on body 50. In the case of stationaryclamps 14, the clamping action is on the plate alone—between tip 40 anddrum surface 20.

It is noted that the construction of the clamps of the presentinvention, of both types, is inherently simple and involves a veryminimal number of components. Such clamps are therefore advantageouslyinexpensive to fabricate.

Typical operation of the clamps will now be explained with reference toFIG. 6. There is shown, by way of example, an activation mechanism,positioned at a plate loading station, in relation to the drum and theclamps thereon. It includes a bar 64, subtending the length of drum 10and suspended, parallel to its axis, at a certain distance from surface20. In bar 64 are pins 66, one pin essentially coplanar with each groove12, which are movable radially. The pins may be moved by any of avariety of means, such as solenoids, pneumatic actuators or a cam. In analternative configuration, bar and pins may be replaced by a directactivation mechanism, such as a cam or a manually operable leverassembly. Mounting of a plate, to be subsequently plotted, onto drum 10typically proceeds as follows:

1. Initially all slidable clamps are preferably aligned along a commonline, parallel to the drum's axis and circumferentially just far enoughfrom the stationary clamps to freely accommodate the full length of theplate; the latter condition can be arrived at by moving all slidableclamps in a manner similar to that described in steps 7 and 8 herebelow,whereas initial alignment along a line can easily be performed manually.

2. The drum is rotated to a position (shown in the illustration) suchthat notch 36 on body 34 of each stationary clamp 14 is directly under acorresponding one of pins 66.

3. Pins 66 are pushed inwards, pressing on bodies 34 against the forceof spring 38 (FIG. 5) and causing tips 40 to lift off surface 20.

4. A new plate (not shown) is introduced tangentially to the drum (fromtop right, in the illustration) and registered thereon (preferably bymeans of registration pins provided thereon) so that preferably itsleading edge is positioned under tongues 36 of all relevant clamps 14, acertain distance behind corresponding tips 40.

5. Pins 66 are withdrawn, freeing all grippers 26 to grip the edge ofthe plate, in a manner explained hereabove.

6. The drum is slowly rotated forward (counter-clockwise in theillustration) while the plate is being gradually wrapped around itssurface, for example by means of rollers (not shown); if a vacuum systemis included, the vacuum may be applied at this step.

7. Rotation stops when notches 58 on bodies 50 of slidable clamps 16 arepositioned directly below pins 66; the pins are then pushed inwards,pressing on bodies 50 against the forces of springs 51 (FIGS. 4) andcausing tips 60 to lift off surface 20; they also cause bases 44 to bepushed against the bottom surfaces of grooves 12

8. The drum is slowly rotated backwards (clockwise in the illustration)while clamps 16, which are held stationary by pins 66 engaging notches58, slide along their respective grooves 12.

9. The backward rotation stops when the other (preferably trailing) edgeof the plate becomes positioned under tongues 52 of the relevant clamps,a certain distance behind corresponding tips 60.

10. Pins 66 are withdrawn, freeing all grippers 46 to grip the trailingedge of the plate, in a manner explained hereabove.

11. The drum is accelerated to normal operational speed and plottingproceeds in the usual manner.

Demounting of the plate, after plotting, proceeds in a similar manner,though essentially in a reverse order. For this operation, another bar,similar to activation bar 64 with pins 66, but positioned at anunloading station, may be used.

It is noted that during normal (fast) drum rotation, the clamps firmlygrip the plate, through the pressure of the tips of the tongues againstthe drum surface, and simultaneously each slidable clamp is, in effect,anchored in its position, through friction (preferably by means of thefriction pads) between the top surface of its base and the top surfaceof the wide portion of the groove—all assisted by centrifugal effects,as explained hereabove.

It is also noted that the position of each slidable clamp along itsrespective groove is infinitely variable, within a range determined bythe circumferential length of a groove, and that therefore the length ofa plate that can be gripped by it may assume an infinite number ofvalues within a corresponding range. It is likewise noted that operatingthe clamps (both in gripping a plate and in positioning a slidableclamp) is relatively easy, whether done manually or by an activationmechanism such as described hereabove. It is further noted that even ifonly a single length of plates need ever be accommodated, theconfiguration with slidable clamps advantageously eases the process ofgripping plates during mounting and releasing them during demounting(all—by the use of reverse rotation).

It will be appreciated that other means and procedures, including manualones, for mounting a plate on the drum, or demounting it therefrom, andfor operating the clamps and moving the slidable clamps along thegrooves, are possible—all coming within the scope of the presentinvention. It will also be appreciated that a drum may be provided withany number of stationary clamps and any number of grooves, each groovewith one or more slidable clamps, but not all clamps need be engaged; inparticular, if a plate substantially narrower than the length of thedrum is mounted, only a corresponding number of the clamps would beengaged in gripping it. Although, in the preferred embodiment, eachstationary clamp is paired, and axially aligned, with a groove and acorresponding slidable clamp, it will be appreciated that such pairingand/or alignment is not essential and that the number of stationaryclamps need not be equal to the number of slidable clamps. In such acase, the activating mechanism of FIG. 6 may include additional pins—forany stationary clamps that are not aligned with corresponding grooves.It is noted that, with a plurality of clamps and grooves, it is possibleto mount a plurality of plates, side by side along the drum; moreover,the plates thus mounted may variably be of any width (limited, ofcourse, by the total width having to be less than the drum's length) andof any length (within the maximum length that can be accommodatedbetween the clamps). Furthermore, in the case of multiple slidableclamps in any one groove, these may be paired with each other, clamps ofany one pair facing each other and operable to hold a plate betweenthem; in particular, if their number is odd, one slidable clamp is madeto face the stationary clamps, while the others are mutually paired;such a configuration is similar to that described in the next paragraph.

Another configuration is shown schematically, in an end view of thedrum, in FIG. 8. This configuration is similar to that of FIG. 6, exceptin that all clamps are of the slidable type. Here are disposed in eachgroove (not shown) around drum 10 a pair of slidable clamps 16, with thetips 60 of each pair circumferentially facing each other. The operationof this configuration is similar to that of FIG. 6; however, in thisconfiguration there is freedom in choosing the set of clamps to beoperated first during plate mounting and, independently, the set ofclamps to be operated first during plate demounting. This freedom isadvantageous in the context of the loading and unloading arrangement, aswill be explained herebelow.

A variation of the configuration of FIG. 8 is shown in FIG. 9. Here aredisposed in each groove around drum 10 two pairs of slidable clamps 16.It is then possible to mount two plates around the drum's circumference,such as plates 18, whereby each plate may independently have any length(subject, of course, to the total of the lengths not exceeding thatwhich can be accommodated between the clamps at maximal intra-pairseparation). This configuration advantageously provides a high degree ofequipment utilization, while permitting flexibility in plate sizes. Itis noted that the two plates may also be of different widths.Furthermore, as with the previously described configurations, it is alsopossible to mount additional plates side by side, by providingsufficient grooves (at least two per plate across) and correspondingpairs of clamps. Thus, for example and as illustrated in FIG. 10 (whereonly half of the clamps are viewable), it is possible to mount fourplates 18, all of the same or different sizes, in a generally two acrossby two circumferential arrangement, using eight pairs (or more) ofslidable clamps 16 in four (or more) grooves 12. Clearly, the number ofpairs of clamps in each groove can also be greater than two—allowingmounting more than two plates circumferentially. It is appreciated thatalso in the configurations of FIGS. 8 and 9 one set of clamps may be ofthe stationary type, whereby one slidable clamp in each groove isreplaced by a fixed clamp; this case will, however, not manifest theadvantage of the flexibility in loading arrangements, explainedherebelow.

It is noted that the arrangements of slidable clamps discussed hereaboveand illustrated in FIGS. 8-10 are also possible if the clamps are not ofthe dynamic type, as described herein (that is—if the clamping force isnot derived from centrifugal action), or of a different dynamic typethan that disclosed herein. In other words, any clamps that are movablealong respective tracks and fixable at a plurality of positionstherealong would enable attaching multiple plates of various sizes andwould thus come within the scope of the present invention. Moreover, theclamps need not be isolated clamps, each moving along its own track andoperable separately, as described herein, but may, for example, also begrouped into assemblies of clamps, each assembly joined by a bar andmoving and operable in common, as sketched in FIG. 14B. Another exampleof clamps admissible by the present invention is sketched in FIG. 14A,showing a single wide clamp, attached to the drum at a plurality oftracks; alternatively, there may be a plurality of such wide clamps—onefor each clampable plate across the drum. It is noted that FIGS. 14A and14B show only one clamp along each circumferential groove, its matchingclamp (as well as circumferentially additional pairs of clamps) beinghidden from view. The dynamic slidable clamps disclosed herein, such asclamps 16 in FIG. 4, are, however, particularly advantageous since theyprovide strong clamping force at any speed of drum rotation andrelatively easy and convenient way of releasing them and of moving themalong their grooves, as well as fixing each clamp at any of an infinitenumber of positions along its track (i.e. groove); furthermore, theirbeing operable individually allows grouping of adjacent clamps toaccommodate plates of various widths (whereby, for example, two clampsacross would be used for a narrow plate and three or more clamps acrossfor a wider plate).

The operations of mounting and demounting plates on a drum with slidableclamps, and its relation with loading- and unloading stations will nowbe further explained with reference to

FIGS. 11A, 11B, 12A and 12B, which are schematic end views. There isshown schematically in FIGS. 11A and 11B an exemplary arrangement of aplate feeder 72 and a first examplary arrangement of a plate stacker 74,in relation to drum 10 and to activation bar 64, associated with aloading station 71, and to activation bar 64′, associated with anunloading station 73. As shown in these views, plate feeder 72 isgenerally disposed above and to the right of the drum, while platestacker 74 is below and to the left of the drum. For loading, depictedin FIG. 11A, a plate 18 is fed from feeder 72 (manually or by amechanism, not shown), extended leftward, tangentially to drum 10, andmounted onto the drum according to the enumerated procedure hereabove.In particular, its leading edge 76 is clamped to drum 10 by a first rowof clamps 16, when released by activation bars 64 (steps 4 and 5) andthe drum then rotates slowly counterclockwise (step 6) until the plate'strailing edge 78 reaches the vicinity of loading station 71, at whichpoint it is clamped to the drum by a second set of clamps 16′, which areslidable (steps 7-10). For unloading, illustrated in FIG. 11B (which issimilar to 11A, except that the drum is shown in a differentorientation), the second set of clamps, 16′, is first brought tounloading station 73, where they are released by activation bar 64′,then, through slight CCW rotation of the drum, trailing edge 78 of theplate is freed from the clamps. The drum is then slowly rotated CW,pushing the plate (with its trailing edge, 78, now leading) left towardstacker 74, until the first set of clamps, 16, reaches unloading station73. At that point the clamps are released, by activation bars 64′, andthe plate becomes free to be pulled (manually or by a mechanism, notshown) onto stacker 74.

It is noted that in the configuration of FIGS. 11 the first set ofdynamic clamps (16) may be of the fixed type, but the second set (16′)must be of the movable type. It is further noted that in thisconfiguration the first set of clamps (16) may be of any fixed type, notnecessarily of the dynamic type (as described hereabove) and thus wouldbe activatable and releasable by a mechanism other than activation bar64. This configuration, which is typified by CCW drum rotation duringloading and CW rotation during unloading (or vice versa), has somedisadvantages for the overall design and operation of the machine, sinceeither the feeder or the stacker may thus be difficult to reach by themachine's operator. In an alternative arrangement of this configuration,the feeder and stacker are placed near each other, e.g. both at the topright, and thus may be more conveniently reachable, but they presentmutual space constraints.

An alternative preferable, and generally more advantageous,configuration of plate loading and unloading is such that bothoperations occur with the drum rotating in the same sense (either CW orCCW). Two exemplary arrangements of this configuration are illustratedin FIGS. 12A and 12B, in both of which plate loader 72 is, again, aboveand to the right of drum 10. In the arrangement of FIG. 12A stacker 74is below and to the right of the drum, while in the arrangement of FIG.12B stacker 74 is above and to the left of the drum. Clamp activationbars 64 and 64′ are, again, positioned with respect to theircorresponding stations. Under this configuration all dynamic clamps mustbe of the movable type and are preferably disposed in circumferentiallyrunning grooves. Operation is similar for all arrangements. Loading andmounting operation is identical to that under the configuration of FIG.11A, as described hereabove, using activation bar 64, with platemounting occurring under CCW drum rotation. However demounting andunloading proceeds as follows: First, the drum is rotated to place thefirst set of clamps (16) under activation bar 64′, which is thenactivated to keep them open. The drum is then rotated slightly CW, torelease leading edge 76 of the plate from under clamps 16. Thereafter,the drum is rotated CCW, letting leading edge 76 of the plate proceedtoward stacker 74. When the second set of clamps 16′ becomes positionedunder activation bar 64′ (which is the situation depicted in FIGS. 12Aand 12B), the drum stops and the mechanism is activated to release theclamps, whereupon the plate is pulled away to stacker 74.

It is noted that the arrangement of FIG. 12A may be advantageous when itis desirable to access both the loader and the stacker from one side ofthe machine. The arrangement of FIG. 12B may be advantageous when it isdesirable to access both the loader and the stacker from above themachine. The latter arrangement has a further advantage in that a singleactivation bar may be used for both the mounting and the demountingoperations.

It is also noted that, for proper operation of the configuration of FIG.12, slidable clamps 16 and 16′ need not be mounted in a groove, as inthe preferred embodiment, but could be circumferentially movable in anyother way; such other ways could also be associated with a clampactivation and—release mechanism that is different from activation bar64 (or 64′) and could even be attached to the drum. However, if theextent of such motion is limited, the other advantages of the preferredembodiment, namely the ability to accommodate plates of various lengths,would be lost.

It is further noted that slidable clamps 16 and 16′ need not be of thedynamic type (i.e. with clamping force derived from centrifugal action),as described further above or of any other dynamic type, nor need theybe activated or released by an activation bar, such as bar 64, to comewithin the scope of the present invention. However, dynamic slidableclamps, as disclosed herein, are particularly advantageous, since theyare of simple design, yet exert strong clamping force and are easilymovable during loading and unloading, especially in conjunction with anactivation bar as described. It will be appreciated that theconfigurations described hereabove may be similarly operated for eachplate when a plurality of plates are to be mounted on the drum, whetherin a circumferential sequence or side-by-side, as described hereabove.

It is still further noted that additional pairs of clamps may be mountedaround the drum, such as clamps 17 and 17′, shown in FIGS. 11A to 12B.Such clamps may be used to attach additional plates around the drum,using the same respective loading- and unloading mechanisms andprocedures as those used with respect to clamps 16 and 16′, as describedhereabove.

In other alternative configurations of dynamic clamps on the drum, whichare basically similar to ones described hereabove, there may beadditional clamps, preferably of the slidable type, disposed on the drumso that they can operate to grip a plate 18 along its side edges. Apreferred embodiment of such a configuration is illustrated in FIGS. 7and 13. Here a number of grooves 12′, similar to grooves 12, are cutinto surface 20 of drum 10 in a direction generally parallel to thedrum's axis. Pairs of slidable clamps 16′, similar to 16, are mounted ingrooves 12′, the tips of members of each pair facing each other. One ormore plates 18 are mounted on the drum and each held, at one end, bystationary clamps 14 or slidable clamps 16 and, at the other end, byslidable clamps (not shown). Slidable clamps 16′ are then operated, in amanner to be described, to grip the side edges of a mounted plate andthus offer additional safety and holding power; they may be particularlyadvantageous in holding down a plate on a drum that lacks a vacuumsystem. It is noted that plates of any width (up to nearly the length ofthe drum) may thus be gripped.

Activation of clamps 16′ is effected by means of correspondingactivation mechanisms 64″, shown in FIG. 13, which are similar toactivation bar 64, except that they are movable parallel to the drum'saxis. In order to activate several clamps simultaneously, a number ofsuch mechanisms may be disposed at appropriate mutual circumferentialdistances around the drum. Moving a clamp 16′ within its groove is theneffected by bringing the corresponding groove (through drum rotation) tounder an appropriate pin 66, then moving the respective activationmechanism 64″ until its pin 66 engage notch 58 of the appropriate clamp16′, then causing (FIG. 4) pin 66 to push the body of the clamp and tothus release it and further moving the respective activation mechanism64″, to slide the clamp onto or off the edge of plate 18, as the casemay be.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made.

We claim:
 1. Apparatus for clamping flexible plates of various sizes onto the cylindrical surface of a rotatable drum, comprising: a rotatable drum having a cylindrical surface; a plurality of grooves, cut into the surface of the drum; at least one clamp, each attached to the drum at one or more corresponding grooves and being switchable between two states—a clamping state and a released state; and at least one activation mechanism, not attached to the drum and including at least one member that is movable essentially radially to the cylindrical surface; wherein each of said clamps is fixed at any of a plurality of positions along the respective grooves and operative to clamp an edge of one of the plates, which crosses the respective grooves, to the surface of the drum, when in a clamping state, and is slidable along the respective groove, when in said released state, and wherein said at least one member of said at least one activation mechanism is operative: to engage any clamp, while said clamp is in an appropriate position, to switch said clamp to said released state, by moving radially inwards, and to switch said clamp to said clamping state, by moving radially outwards.
 2. The Apparatus of claim 1, wherein each of said clamps is a dynamic clamp, whereby the force at which it clamps the plate, when in the clamping state, consists of a constant component and a dynamic component, which is derived from centrifugal action and is essentially proportional to the rotational speed of the drum.
 3. The Apparatus of claim 2, wherein each of said clamps includes a base that is slidable within, and along, a respective one of said grooves and forming one member of a hinge, having a pivot axis, and a gripper, formed to have a tip and operative to press the edge of one of the plate against the drum, the gripper being attached to said base so as to form the other member of said hinge and so that the center of mass of said gripper is at a substantial distance from said pivot axis and in a first direction therefrom that is generally opposite the direction of said tip from said pivot axis.
 4. The Apparatus of claim 3, wherein said at least one member is operative to engage said gripper of any clamp, while in an appropriate position, at a point thereof that lies in said first direction from said pivot axis.
 5. The Apparatus of claim 4, wherein said grooves lie along circumferential lines and wherein said at least one activation mechanism and any of said clamps, while in said release state and while being engaged by said at least one member, cooperate to cause said base of the clamp to slide along the respective one of said grooves in reaction to a rotation of the drum.
 6. The Apparatus of claim 4, wherein said grooves lie along lines parallel to the axis of the drum and wherein any of said members of said activation mechanism is movable parallel to the axis of the drum and is cooperative with any of said clamps whose gripper is engaged by said at least one member to cause the base of the clamp to slide along the respective grooves.
 7. The Apparatus of claim 1, wherein said grooves lie along circumferential lines.
 8. The Apparatus of claim 7, wherein the number of said grooves is at least four, the apparatus being operative to clamp at least two plates, side by side, onto the surface of the drum, the plates not necessarily being equal in length or in width.
 9. The Apparatus of claim 7, wherein the number of said clamps in any one of said grooves is even, the clamps forming circumferentially adjacent pairs, the clamps of each pair being disposed and operative to clamp respective opposite edges of a corresponding one of said plates.
 10. The Apparatus of claim 9, wherein the number of said clamps in any one of said grooves is at least four, the apparatus being operative to clamp to the drum at least two plates of various lengths in tandem around the drum.
 11. The Apparatus of claim 1, wherein said grooves lie along lines parallel to the axis of the drum.
 12. The Apparatus of claim 1, wherein some of said grooves lie along lines parallel to the axis of the drum and others of said grooves lie along circumferential lines. 